The invention relates to a fuel cell with a proton-conducting membrane, on which catalyst material and a collector are arranged on both sides.
Fuel cells are used for electrochemical conversion of chemical energy, in particular in the form of gaseous hydrogen and oxygen, into electrical energy. Of the large number of known types, so-called PEM fuel cells (PEM=polymer-electrolyte membrane) are preferred, for example for mobile use. The advantages of fuel cells of this type reside in a comparatively low operating temperature (up to about 100.degree. C.), in the absence of a corro-sive liquid electrolyte, in the stability with respect to carbon dioxide (CO.sub.2) and, finally, in a relatively simple mechanical structure. In addition to the cell housing, cooling units or separators, gas supply or distribution means and means for constructing fuel cell stacks from individual elements, PEM fuel cells actually consist essentially of two gas-permeable, porous, electrically conductive collectors on the anode and cathode sides, which are next to the solid-electrolyte membrane.
Between the collector and the membrane, there is in each case a catalyst in finely divided, catalytically active form, for example platinum or a platinum alloy. One side of the fuel cell is supplied with a combustible gas in particular hydrogen or a hydrogen-containing gas, and the other side is supplied with an oxidant, in particular oxygen or an oxygen-containing gas, such as air. Hydrogen is oxidized at the anode, protons being produced which diffuse through the membrane to the oxygen side; in this case, water is generally entrained with them (so-called drag effect). At the cathode, the protons recombine with reduced oxygen to form water, referred to as product water, which is removed in suitable fashion from the fuel cell.
Through the drag effect, water is drawn from the anode side of the membrane, so that this side dries out and therefore loses its function if not enough water is added. Further problems are the high costs for production of the membrane, and the lack of cost-efficient processes for producing membrane/electrode units with a low level of catalyst coating and high power density, in particular for operation with air at close to atmospheric pressure. Indeed, for relatively thick membranes, the ohmic losses have a power-reducing effect.
Technical solutions for fuel cells are already known (see, for example, DE-A 33 21 984 and EP-A 0 560 295). The gas-permeable, electron-conducting layers, that is to say collectors, used in this case are carbon paper (U.S. Pat. No. 4,215,183) and carbon fabric ("J. Appl. Electrochem.", Volume 22 (1992), pages 1 to 7); metal structures may also be considered (DE-A 42 06 490). The proton-conducting membranes used are perfluorinated sulfonated polymers such as nafion, raymion and permion ("Ber. Bunsenges. Phys. Chem.", Volume 94 (1990), pages 1008 to 1014). For the sake of convenience, the layer thickness of the membranes is between 50 and 200 .mu.m. Important properties of the membranes are heat-resistance (up to about 100.degree. C.), reduction and oxidation stability, resistance to acid and hydrolysis, sufficiently low electrical resistivity (&lt;10 .OMEGA..multidot.cm) with ion conduction (H.sup.+) at the same time, low hydrogen or oxygen permeation and freedom from pin-holes. At the same time, the membranes should be as hydrophilic as possible in order, through the presence of water, both to ensure proton conduction and (by reversed diffusion of water to the anode) to prevent the membrane from drying out and therefore to prevent a reduction in the electrical conductivity. In general, properties of this type are achieved with materials which have no aliphatic hydrogen-carbon bonds, which, for example, is achieved by replacing hydrogen by fluorine or by the presence of aromatic structures; the proton conduction results from the incorporation of sulfonic acid groups (high acid strength).
The electrodes, that is to say the catalyst layers arranged between the collectors and the proton-conducting membrane are essential for correct operation of a fuel cell. On these layers, which consist of very finely divided catalyst material which, for example, may be applied to carbon, the fundamental processes take place, namely adsorption, dissociation and oxidation of hydrogen on the anode side, or the corresponding reduction of oxygen on the cathode side. The layers must have sufficient gas permeability and catalytic activity, that is to say a large internal surface area, the intention being for the amount of catalyst, for example platinum, to be as small as possible for economic reasons. As an example, fuel-cell electrodes currently require amounts of platinum of between 3 mg/cm.sup.2 (EP-A 0,560,295) and 0.095 mg/cm.sup.2 (EP-A 0,569,062) or 0.07 mg/cm.sup.2 ("J. Electrochem. Soc.", Volume 139 (1992), pages L28 to L30). In order to ensure intimate contact between the collector, electrode (=catalyst) and membrane, the layers are usually compressed at high temperature. The housings of the individual fuel cells are configured in such a way that a good gas supply is ensured, and at the same time the product water can be discharged properly. In order to obtain sufficient power, fuel cells are usually joined to form stacks, the requirements which have been mentioned being met through the design.
Although it is actually known to achieve internal wetting through thin membranes (WO 92/13365), this is limited by the minimum handlable layer thickness (&gt;50 .mu.m). In addition, it is already known (U.S. Pat. No. 5,242,764) to apply thin membranes (&gt;20 .mu.m) by wet chemical means to electrodes, and subsequently compress them (total thickness &gt;40 .mu.m). However, this procedure is restricted because of the wet chemical method, for example in terms of layer thickness and material losses in the process, and, in addition, there is no indicated solution as to how the requirements for planarity on the collector surface, in particular for relatively thin membranes, can be met. Furthermore, the electrode which is used has a platinum coating level of 1 mg/cm.sup.2 and is therefore a long way from meeting the requirements in terms of high power density and low cost. In addition, the membrane/electrode unit is sealed at the edge by a membrane, having a central opening, which overlaps this unit. However, this is very difficult and intricate to carry out, because the sealing membrane must likewise be very thin; furthermore, gradation of the membrane is very expensive.
Yet other problems can occur with current technology. Thus, for example, collectors made of graphite paper or carbon fabric, even when they are compressed at high pressure, sometimes only have point contact with the catalyst material. It then becomes difficult for the electrons to flow from the electrode to the collector. The membranes are currently produced using conventional wet chemical methods (polymerization, sulfonation), but this necessarily leads to waste disposal problems and to environmental pollution.